A boiler employed in the power plant of a vehicle, such as an automobile, must quickly respond to throttle settings that change rapidly and over a wide range. For example, the rate of heat input may change from 50,000 BTU's per hour to one million BTU's per hour in a period of time in the order of magnitude of three seconds. It is therefore of the highest importance to provide sufficient water to the boiler so that the desired final temperature is maintained in the face of the rapidly changing heat input and to prevent any part of the boiler from becoming too hot or too cold during transition from one steady state condition to another. Because of its simplicity, a monotube boiler is ideally suited for automotive use. A typical boiler of this type may contain approximately one quart of water at any time so that when the throttle setting is changed rapidly, as during acceleration, and the heat input increases rapidly, sufficient water must be furnished to the boiler so that the boiler tube temperatures do not become excessive. Furthermore, it is necessary to add water in such precise amounts that overcontrolling and hunting about the new set point do not occur.
It is the function of a temperature control apparatus associated with a monotube boiler of this type to provide the requisite degree of control. An object of the invention, therefore, is to provide a new and improved temperature control apparatus which achieves this result and at the same time utilizes relatively simple components, is very sensitive to changes in temperature in the boiler tube, and acts rapidly in response to such changes to provide the necessary temperature control.